Hepatitis C virus (HCV) RNA is thought to be less stable than HCV core antigen (HCV-Ag), however there have been few studies on comparing the stability of HCV-Ag with that of HCV-RNA in vitro. The aim of this study is to evaluate serial levels of HCV-Ag and HCV-RNA in serum before and after incubation at 4 or 25 degrees C for 7 days to estimate an assay suitable for general laboratory use. In this study, we demonstrate that HCV-Ag levels are highly reproducible (coefficients of variation (CVs); 0.89-6.92%) and stable (84.8% of the initial level) with incubation of even 25 degrees C for 7 days, whereas HCV-RNA levels are much less reproducible (CVs; 9.13-29.66%) and decrease dramatically (15.1% of the initial level) after incubation, particularly at 25 degrees C. The measurement of the HCV-Ag level was found to be suitable for HCV quantification with serum samples stored either at 4 degrees C or under unknown conditions. Additionally, it successfully eliminated inhibitors such as heparin from plasma and could be applied to a variety of clinical specimens. Our data suggest the significance of measuring the HCV-Ag level during clinical management independently of the HCV-RNA level, particularly because of its high stability.